Snowballs of bloom followed by miniature fruits tell of branches soon to be bowed by the weight of large and luscious peaches, apples, pears and plums.
Large and luscious, that is, if you pluck some of those fruits now, before they get much bigger.
Fruit thinning, as this operation is called, channels more of a tree’s attention to the fruits that remain. The tree’s goal, after all, is to make as many seeds as possible, secured by ripening lots of fruit, even if the fruits end up small and not as tasty as they could be. What we want, on the other hand, are big, delicious fruits, which come from coaxing the tree to put more energy into fewer of them.
For perfect ripening, for example, each apple fruit needs the energy resources of 20 to 40 healthy leaves, or a dozen in the case of more energy-efficient dwarf trees.
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Another reason you might see me out in the garden these days plucking excess fruitlets from my fruit trees is to make them bear more consistently. My Macoun apple knows no moderation: It wants to ply me with fruits one year, then starve me the next, feast following famine year after year. But I want to bite into those delectable Macouns every year.
Macoun and some other fruits get into this feast-and-famine habit because fruit seeds produce a hormone that suppresses flower bud formation. Most fruit trees develop flower buds the year before the buds actually open, so abundant fruit one year means less fruit the following year.
The most obvious benefit of fruit thinning is just leaving a branch with less fruit on it: Less fruit means less weight, a consideration when that weight might otherwise break a branch.
Putting space between fruits also has some effect on pests. Codling moth caterpillars — the “worm” in an apple or pear — prefer to tunnel into fruits that are touching each other.
Putting space between fruits also helps each one to bask in air and sunlight, both of which hasten drying and so make it harder for diseases to gain foothold.
HOW TO THIN
So much for theory. Now comes the hard part: actually bringing yourself to remove these promising little fruitlets. I just grit my teeth and snap them off with my fingers or use a small clippers, taking care not to damage the knobby little stem to which the fruit stalk is attached. That stem, in the case of apple, pear and plum trees, is the origin of flowers and fruits in years to come.
Gardeners with large trees or little patience can thin fruits by blasting branches with a stream of water, batting branches with a piece of hose slipped over the end of a broom handle, or brushing branches with a stiff brush. (Fruit farmers have chemical sprays that accomplish this same task on a larger scale and more delicately.)
No matter how you do your thinning, you should leave the largest and healthiest fruits.
Many small or damaged fruits eventually fall off anyway. Leave a space between fruitlets that is two to three times the diameter of the mature fruit.
The earlier that fruit thinning is done the better, especially with apples and less so with peaches, then pears. Fortunately, thinning is unnecessary with smaller fruits, such as cherries and European plums. How tedious that would be!
Besides what you and I do now, other fruit thinning will occur and has occurred. Although fruit trees are determined to set as many fruits as possible right after blossoming to increase the likelihood that more will make it through any post-bloom frosts, these plants do have some sense. A few weeks after bloom, once the weather settled, these trees realized how taxing it would be to mature all those fruits. So they shed some. Not enough for our tastes, though.
Fruits – potential fruits – also were thinned earlier in the season, when you pruned your fruit trees. Pruning branches removed dormant flower buds that could have become fruits, and that’s one of many reasons to prune fruit trees annually.
For more about pruning fruit trees, including fruit thinning, see my “The Pruning Book” (Taunton Press, 2010).